Storage of crude oil at sea at present usually takes place at sea surface, on floating storage vessels, in loading buoys with storage capacity, or on the production unit itself in form of a platform or a ship.
Floating production platforms with delivery of the crude oil by use of a tanker is dependent of having storage capacity on the platform or nearby in form of a storage ship or loading buoy with storage capacity, from where the oil can be transported further for example to a land terminal.
Platforms fixed to the seabed have in some embodiments a large fundament that also is comprising storage for produced oil.
The storage for the produced oil is in many respects functioning as ballast, and out of consideration to stability and mechanical loads the oil may have to be replaced with ballast water upon unloading to a tanker. Usually seawater is let into a tank simultaneously as oil is pumped out. By filling of the storage seawater is likewise displaced or pumped out after having passed an oil-water-meter that controls the oil contents of the water before it is discharged. Maximum allowed oil contents of the water to be discharged varies, but is most commonly at present 40 ppm. In connection with storage in the above-mentioned way an undesirable emulsion layer will be formed in the oil load, including oil, water and chemicals. Chemicals are added to keep oil and water better separated. Further, the ingress of relatively small amounts of seawater into the oil load results in undesired salt contents and thereby significantly reduced value of the load. With a water-oil-storage of the above-mentioned type a usual problem is undesired bacterial growth (termed SRB), which in combination with water results in sulphuric acid. It is known that said bacterial growth has resulted in severe problems for tank and pipe systems both on platforms and buoy loading tankers.
The above-mentioned emulsion layer has tendency to gain thickness, and the storage capacity is therefore reduced over time. The emulsion layer can be pumped over to a tanker and transported to a land terminal or refinery to separate water, oil and chemicals and render hazardous components harmless. Said separation and rendering harmless of hazardous components are very expensive and only very few refineries or others undertake such jobs.
A demand exists for providing storage of crude oil or other fluids without the above-mentioned problems. Further, a demand exists for providing a flexible storage for crude oil in such a way that storage units can be transported to where a demand for storage exists, adapted to the actual demand, and allowing production or continued production from marginal fields. A demand also exists for storage without danger for collision with surface vessels, ice or drifting objects.